This invention relates to a method of preparing preforms for optical fibers and, more particularly, to a method of varying the refractive index of the central portion of an optical preform.
Modified Chemical Vapor Deposition (MCVD) is a widely-used process for fabricating preforms wherein successive layers of cladding and core material are deposited on the inside surface of a substrate tube, which is made from high quality glass. Individual layers of deposited material are turned into glass (vitrified) by a torch that moves back and forth along the length of the tube. The core material generally includes germanium, which raises the index of refraction of the glass according to the amount of germanium present. In this manner, a refractive index profile can be shaped to guide lightwaves in the axial direction of the optical fiber that is made from the preform. Different refractive index profiles affect the guided light in different ways.
Axial variations in the refractive index profile are difficult to control in optical fiber preforms, and frequently cause the optical fibers drawn therefrom to depart from their design specification. For example, multimode fibers include a central region of radius (a), referred to as the core, where substantially all of the propagating light is confined. Within the core region, the magnitude of the refractive index (n) varies as a function of radial distance (r) from the center of the fiber according to the equation n=no[1xe2x88x922xcex94(r/a)xcex1]xc2xd. Ideally, the magnitude of xcex1 remains constant over the length of the fiber, but this is not the case. For a variety of reasons including variations in the composition of the precursor gasses during the deposition of core material, xcex1 varies along the length of the preform and the refractive index profile of the fiber is changed accordingly. Multimode fiber bandwidth is critically dependent of the value of xcex1, and extremely small fluctuations can reduce the bandwidth by very large amounts. See e.g., Calculation Of Bandwidth From Index Profiles Of Optical Fibers, Applied Optics, Oct. 1, 1979 by H. M. Presby, et al.
U.S. Pat. No. 5,993,899 describes a technique to produce a radial index profile uniformly in the longitudinal direction by varying the velocity of the gas introduction into the deposition tube. However, this involves a detailed understanding of how the gas velocity into the tube influences the deposition process and it requires a method of monitoring and controlling the velocity of the gas into the deposition tube.
A related technique that has been used to reduce the alpha axial trend in multimode fiber is an algorithm that varies the torch traverse speed as a function of axial preform position and core pass number in order to control the amount of glass being deposited at a particular position. This technique is known as the Multiple Traverse Speed tuning algorithm. And while it provides significant improvement for a large portion of the preform, it does not work well at the intake and exhaust regions of the preform.
Accordingly, it is desirable to provide an improved technique for controlling the axial bandwidth and alpha trend along the entire length of an optical preform and to control fluctuations of the core diameter in both singlemode and multimode preforms.
An optical preform having a desired refractive index profile in its longitudinal direction, is fabricated by first making a precursor preform having a central hole in the longitudinal direction that is surrounded by core material having the approximate desired refractive index profile. The central hole is collapsed and then a length of fiber is drawn from the preform. The refractive index profile is measured at various locations along the length of the drawn fiber. Based on the differences between the desired refractive index profile and the measured refractive index profile, calculations are made regarding the amount of core material that needs to be removed at the various different locations. Thereafter, a second optical preform is fabricated that is similar to the precursor preform, and an etchant gas is used to remove the calculated amounts of core material necessary to cause the second preform to have a refractive index profile that is similar to the desired refractive index profile along its length.
In an illustrative embodiment of the invention, the desired refractive index profile is relatively constant along the entire length of the preform, and the refractive index, n, of the core varies as a function of radius, r, in accordance with the equation: n=no[1xe2x88x922xcex94(r/a)xcex1]xc2xd. In the illustrative embodiment of the invention, xcex1≈2 and, hence, the profile is approximately parabolic. Moreover, since bandwidth of the fiber is related to the xcex1 value, then measurements of bandwidth pretty well define the refractive index profile. Accordingly, in the illustrative embodiment, bandwidth measurements of the fiber are made to determine refractive index profile.
Also in the illustrative embodiment, SF6 and oxygen are flowed through the central hole in the second preform to perform etching. The amount of etching performed at various locations along the length of the preform is varied by changing the quantity of SF6 flowing through the central hole and/or the traverse speed of the torch.
Finally, in the illustrative embodiment, a number of precursor preforms are used to determine average refractive index profiles at the various locations and these averages are then used to determine the amount of core material that needs to be removed in subsequent preforms.